fix(dynamic-links, android): null-check intent before calling getDynamicLink (#6911)

Author: mikehardy

packages/firebase_dynamic_links/android/src/main/java/io/flutter/plugins/firebasedynamiclinks/FirebaseDynamicLinksPlugin.java

@@ -209,8 +209,8 @@ private void handleGetDynamicLink(final Result result, Uri uri) {
   }
 
   private void handleGetInitialDynamicLink(final Result result) {
-    // If there's no activity, then there's no initial dynamic link.
-    if (activity == null) {
+    // If there's no activity or initial Intent, then there's no initial dynamic link.
+    if (activity == null || activity.getIntent() == null) {
       result.success(null);
       return;
     }